(1) Field of the Invention
The present invention relates to composite structural members comprising polymers, natural and synthetic fibers, and preferably nano-scale platelets, arranged in two- or three-dimensional cellular skeletal structure; more particularly referring to a low-cost natural-fiber based structural member with material hybridization and material layout for improved behavior that leads to efficient structural beam and plate/panel components that can be used in a plurality of modular structures, resulting in lower cost and reduced environmental impact.
(2) Background of the Invention
Fiber reinforced polymer (FRP) composites have surpassed their initial target applications in the aerospace industry to become a viable material alternative in the sporting goods, automotive, and construction industries. High performance FRP composites made with synthetic fibers such as carbon, glass or aramid embedded in polymeric matrices provide the advantages of high stiffness and strength to weight ratio and increased chemical inertness compared to conventional construction materials, i.e., wood, clay, concrete and steel. In spite of these advantages, the widespread use of synthetic FRP composites has been limited, among several factors, due to their higher initial material costs, their use in non-efficient structural forms and their environmental impact.
Increased environmental awareness and the interest in long-term sustainability of construction materials have thus challenged the development of environmentally friendlier alternatives to synthetic oil-based FRP composites (Mohanty et al. Macromol Mater Eng, Vol. 276/277, 1-24 (2000)). Natural-fiber-reinforced polymer composites, or biocomposites, have emerged in the past decade as an environmentally friendly and cost-effective option to synthetic FRP composites. Despite the interest and environmental appeal of biocomposites, their use has been limited to non-primary, or non-load-bearing applications due to their lower strength and stiffness compared with synthetic FRP composites (Biswas et al., “Development of Natural Fibre Composites in India”, Proceedings of the Composites Fabricators Association's Composites, Tampa, Fla. (2001)). Recent developments, however, have shown that the properties of “engineered” biocomposites (Mohanty et al., “Surface modifications of Natural Fibers and performance of the Resulting Biocomposites: An Overview,” Composite Interfaces, 8, 313-343 (2001); and Mishra et al., Mishra, S., et al., Composite Science and Technology 63 1377-1385 (2003)) are a technical, economical, and environmentally conscious alternative to E-glass fiber reinforced composites (the most common synthetic fiber composite) without sacrificing performance.
While biocomposite materials with specific properties equivalent to entry-level structural materials are feasible, this performance level is still not enough to make them able to compete with existing construction materials. However, the structural performance of a component depends on both its material and structural properties. The lower material stiffness of biocomposites can thus be overcome by using efficient structural configurations that place the material in specific locations for highest structural efficiency. Cellular and sandwich structures are structural configurations that yield high structural performance for minimum material use and thus minimum weight (Gibson, et al., Cellular Solids: Structure and Properties. Pergamon Press, Oxford (1988)). This concept has been recognized for some time and has recently gained new attention due to the way that natures own materials and structures follow these principles.
Natural fibers embedded in a natural or synthetic polymeric matrix, known as biocomposites, have gained recent interest because of their low material and manufacturing costs, light weight, high specific modulus (elastic modulus over density), and environmentally friendly appeal (Mohanty et al., Macromol Mater Eng, Vol. 276/277, 1-24 (2000)). Natural fibers are categorized depending on their source as either leaf or bast fibers. Bast fibers have the highest mechanical properties and thus are ones typically considered for structural applications. The most common bast fibers are flax, hemp, jute, and kenaf. Typical mechanical properties of these fibers together with E-glass fibers are given in Table 1. All natural fibers are lingo-cellulosic in nature with the basic components being cellulose and lignin. The density of natural fibers is about half that of E-glass (Table 1), which makes their specific strength quite comparable, while the elastic modulus and specific modulus is comparable or even superior to E-glass fibers.
TABLE 1Mechanical Properties for Selected Natural Fibers and E-Glass FiberElasticTensileSpecificSpecificFiberDensityModulusStrengthModulusStrengthType(g/cm3)(GPa)(MPa)(GPa/g/cm3)(MPa/g/cm3)E-glass2.55732000–350029 780–1370Hemp1.487069047466Flax1.460–80 345–110043–57250–785Jute1.4610–30400–750 7–21275–510Sisal1.3338450–64029340–480
The applications for which biocomposites have been studied include interior paneling of automobiles (Biswas et al., Proceedings of the International Conference and Exhibition on Reinforced Plastics, Indian Institute of Technology, Madras, 26-36 (2002)) and replacement of wood in housing applications such as plywood, roof surface paneling, partitioning and furniture (Biswas et al. Proceedings of the Composites Fabricators Association's Composites, Tampa, Fla. (2001)). The current market uses of natural fiber composites in North America are shown in FIG. 1 (Mohanty et al., Macromol Mater Eng, Vol. 276/277, 1-24 (2000)). However, the uses in these markets have been limited to non-structural applications where weight and cost can be reduced (Biswas et al., Proceedings of the Composites Fabricators Association's Composites, Tampa, Fla. (2001)).
Consideration of biocomposites for load-bearing, or structural applications has been neglected due to their low stiffness and strength in comparison with conventional construction materials, and only limited research and development projects have considered potential structural uses (Scott, C. T., et al., Wood and Fiber Science, 27(4): 402-412, (October 1995); and Shenton III, H. W., et al., “Manufacture of Fiber-Reinforced-Foam Composite Sandwich Structures”, Proceedings of the International Conference on Advances in Building Technology, Hong Kong, China (December 2002)). However, recent research on biocomposites has shown that “engineered,” or treated, natural fibers can lead to biocomposites with properties that can compete with glass fiber composites (see FIG. 2). This has motivated further research initiatives that consider biocomposites as a technical and environmentally conscious alternative to E-glass reinforced fiber composite (the most common synthetic fiber composite) without sacrificing performance. Natural fiber unsaturated polyester composites show lower density, equal flexural modulus, comparable flexural strength but relatively poor impact strength as compared to a glass fiber composite as shown in FIG. 2 (Mishra et al., Polymer Composites, 23, 164-170 (2002), Mohanty et al., Journal of Polymers and the Environment, Vol. 10, Nos. 1/2, (April 2002)).
Increased environmental awareness and interest in long-term sustainability of material resources has motivated considerable advancements in composite materials made from natural fibers and resins. Natural and wood fiber plastic composites are among the most rapidly growing markets within the plastics industry. Applications for these composites range from building products, automotive, and consumer/industrial applications. However, despite the developments on biocomposites technology and the many applications thus far, their lower stiffness and strength properties have limited their applications to non-load-bearing components. Technical developments on natural fiber-reinforced composites, or biocomposites, have shown that they are a technical, economical, and environmentally conscious alternative to E-glass fiber reinforced composites (the most common synthetic fiber composite) without sacrificing performance. In spite of these achievements, biocomposites are hindered by the hydrophilic nature of the natural fibers and the heat susceptibility of the polymer matrix. In addition, while the properties of biocomposites can compete with those of E-glass composites, their strength, and particularly stiffness, is smaller than that of conventional structural materials.
Development in biobased polymers in our group has shown that blending of functionalized soybean oil with petro-based resins can increase the toughness of a petroleum-based thermoset resin without compromising stiffness and improving its environmental friendliness (Belcher, L., Polymeric Materials Science and Engineering, American Chemical Society, 87, 256-257 (2002)). Emerging research on layered silicate polymer composites, consisting of nano-clay sheets within a polymer, have shown that a small amount (˜1-2%) of nano-scale layered silicate particles can significantly enhance and stabilize the mechanical and thermal properties of the base polymer and improve its fire retardancy (Garcés, J. M., et al., Advanced Materials, Vol. 12, no. 23, pp. 1835-1839 (2000); and Vaia, R., “Polymer Nanocomposites open a New dimension for Plastics and Composites,” The AMTIAC Newsletter, Vol. 6, No. 1) without sacrificing viscosity. The true value of layered silicate composites is not solely the enhancement of the neat resin but rather the value-added properties it provides to the fiber-reinforced composite. Thus, natural fibers must remain as the predominant reinforcement for providing stiffness and strength. Finally, scrutiny of nature's materials shows that high structural efficiency can be achieved by optimized hybrid designs that efficiently combine constituents and material arrangement, or shapes, that maximize performance with as little material as possible, i.e., sandwich structures (Gunderson, S. L., et al., “Natural Cellular and Sandwich Structures for Innovative Design Concepts,” Proceedings of the ASC 8th Annual Technical Conference, Cleveland, Ohio, 431-440 (19-21 Oct. 1993); and Nogata, F., “Learning About Design Concepts From Natural Functionally Graded Materials,” Composites and Functionally Graded Materials, ASME, MD, 80,Dallas, Tex., 11-18 (1997)).
The invention thus focuses on natural-based composites, or biocomposites, that can serve as a sustainable alternative to synthetic load-bearing panels by designing them and manufacturing them in novel sandwich structures obtained from optimized hybrid designs that encompass material constituents, shape and scale effects.
Hybrid biocomposite cellular structures can be used in multi applications (e.g., building walls, floors and roofs, bridge and ship decks, aircraft floors) with tailorable integrated multi-functions (i.e., stiffness, strength, thermal insulation, fire protection, and user friendliness). The sustainability and social acceptance of the proposed components, stemming from its large constituency on rapidly renewable resources, will pioneer the use of agricultural commodities in markets aimed at load-bearing materials and structures.